Raised pavement markers are used as delineators for traffic lanes to alert drivers to roadway changes such as hills, curves, and exit ramps and to improve lane line guidance, especially at night or in poor driving conditions. Some of the many applications for raised pavement markers enable the identification of traffic lane separations, edge lines, fire hydrants, airport taxiways, and other special applications. Raised pavement markers often include a retroreflective lens attached to a marker body. In contrast to mirror-type (or specular) reflection, a retroreflective lens returns light generally directly back to its source. A retroreflective lens appears brightest to observers near the light sourcexe2x80x94a driver and vehicle headlights, for example. This is true for drivers at almost any viewing angle, which makes retroreflective lenses excellent for night visibility. Two common retroreflective lenses used in raised pavement markings include vacuum-metallized retroreflective lenses and totally-internal-reflective lenses.
The vacuum-metallized retroreflective lens is a cube corner prismatic element having a mirror-like metallic surface deposited directly on the surface of the prismatic element. The cubes and mirror-like surface retroreflect light from a headlamp back to the driver of the vehicle. The direct labor and materials used to make this type of lens are relatively inexpensive, but manufacture requires an initial purchase for expensive deposition equipment to form the mirror-like surface. The mirror-like surface absorbs some of the light. Also, moisture that seeps into the lens can corrode the mirror-like surface that further reduces efficiency.
Another type of retroreflective lens is the totally-internal-reflective lens that includes a rigid backing spaced-apart from and behind the cube corner prismatic element to create a hermetically-sealed air gap between the prismatic element and the backing. Under the principles of physics, the refractive index of the prismatic element is chosen such that the air gap causes light entering the prism to be totally and internally retroreflected at the prismxe2x80x94air gap interfaces. Totally-internal-reflective lenses are extremely efficient retroreflective articles. Totally-internal-reflective lenses, however, are often more expensive and difficult to manufacture than vacuum-metallized retroreflective lenses. The rigid backing is often ultrasonically welded or thermally sealed directly to the prismatic elements forming septa that provide for the hermetically sealed air gaps. Generally, totally-internal-reflective lenses are more expensive than their vacuum-metallized counterparts.
Many communities purchase raised pavement markers based on value, i.e., they choose the appropriate raised pavement marker based on a desired performance for a given application. For some communities, however, value must take a back seat to low cost. Because of budgets or other reasons, these communities must settle for low cost markers even when a traffic application demands a better performing marker. Of course, traffic safety is a general human concern and effects everyone. Thus, there exists a need for a low cost, high performance raised pavement marker.
The present disclosure relates to improved raised pavement markers having a totally-internal-reflective lens. The disclosure also relates to methods of manufacturing the raised pavement marker. The raised pavement markers described below include a housing connected to a totally-internal-reflective lens. The totally-internal-reflective lens includes a retroreflective element having a smooth surface generally opposite a plurality of cube corner elements. A film is attached to the retroreflective element at the apexes of the cube corner elements to form spaces, i.e., an air gap, between the film and the cubes. The film and retroreflective element cooperate to form the totally-internal-reflective lens. Light entering the retroreflective element through the smooth surface is retroreflected at the cube/air interface. Methods of manufacturing include, for example, forming a shell with the retroreflective element and attaching the film to the apexes of the cube corner elements.
The raised pavement markers disclosed below include several advantages over other markers, and some of these advantages are described below. One of the advantages is that the markers are high performance but manufactured at a relatively low cost. For example, the totally-internal-reflective lens can be manufactured without septa. Septa, as described above, reduce the surface area that is available for retroreflection. Further, the raised pavement markers disclosed below are significantly more retroreflecting than vapor coated lenses. In a recent laboratory analysis, the retroreflective luminous intensity (measured in millicandellas per lux, or mcd/lx) was found to be 1349 mcd/lx for the markers described below and 487 mcd/lx for the vapor coated lens, each measured with a horizontal entrance angle of zero degrees, an observation angle of 0.2 degrees and a rotational angle of zero degree (in accordance with ASTM-D 4280-96). Likewise, the retroreflective luminous intensity was found to be 849 mcd/lx for the markers described below and 303 mcd/lx for the vapor coated lens, each measured with an entrance angle of twenty degrees, an observation angle of 0.2 degrees and a rotational angle of zero degree (in accordance with ASTM-D 4280-96).